As used herein, the term "blazed diffraction grating" refers to a relief modulation diffraction grating in which the cross sectional shape of the individual periodic grooves is inclined with respect to the entire surface of the grating so that the diffraction grating works more efficiently. For a general description of blazed diffraction gratings, reference may be had to, for example, T. Aoyagi et al, Applied Physics Letters, Vol. 29, No. 5, page 303 (September 1976).
As used herein, the term "relief modulation diffraction grating" refers to a diffraction grating having a periodic rugged structure on the surface as shown in FIG. 8. A relief modulation diffraction grating is contrasted with a refractive index modulation diffraction grating.
The following methods are known for producing blazed relief diffraction gratings:
(a) Directly cutting the substrate plate with a ruling engine.
(b) Drawing directly on the resist coated on the substrate with an electron beam which is changed in dose. Refer to, for example, T. Fujita et al, Opt. Lett., Vol. 7, No. 12, page 578 (December 1982).
(c) Coating the substrate with a photoresist, exposing the photoresist through a mask to form a periodic resist pattern, and carrying out etching aslant with an active or an inactive ion beam using the resist pattern as a mask. Refer to, for example, T. Fujita et al, Applied Physics Letter, Vol. 29, No. 5, page 303 (September 1976).
(d) Exposing and developing a thin resist through the asymmetric double-beam interference exposure method, thereby directly forming a blazed diffraction grating. Refer to, for example, G. Schmahl et al, Progress In Optics, issued by E. Wolf, North-Holland, Amsterdam, 14, page 195 (1976).
The known methods, however, have their respective disadvantages. Method (a) takes a long time for production and leads to a high production cost. Method (b) needs a complex, large apparatus to change the dose, and it is not suitable for drawing a large area. In other words, it can be applied only to the production of small gratings. Methods (b) and (c) need complex, large apparatus, which leads to a high production cost. Method (d) can be used with a simpler apparatus and permits the production of larger gratings than method (b), but it is poor in productivity, which leads to a high production cost. Moreover, method (d) is limited in the shape of the grooves which it can produce
The diffraction gratings produced by the above-mentioned methods are inevitably high in cost because of their poor productivity. While the relief modulation diffraction gratings produced by any of the above-mentioned methods (a to d) may be used to make a mold from which replicas are formed, this process is so complex that it is only possible with a high production cost.